Clean up GetSystemProperty.* APIs

1. Factor out duplicated implementation of
   GetSystemPropertyUint32 on host/linux/android
2. Add GetSystemPropertyBool and factor out the code
   for retrieving bool properties in system/gd/hal/snoop_logger.cc

Test: refactoring CL. Existing unit tests still pass.

Change-Id: I5a24533f2ce6d146e528661e7cf6b86008537f3f